Last week, online storage service Dropbox announced that it was acquiring Mailbox, a hit new iPhone e-mail app. On Thursday, the Gmail-compatible Mailbox said it had reached a million users.

Actually, that's 1 million user registrations. Access to the app download takes a few days, and the Mailbox team has said that it is ramping up the rollout to eventually lose the reservation system completely. In a posting on its blog, Mailbox said it had reached the milestone "only 6 weeks to the day after launching Mailbox."

The company also announced it was releasing an update to its app in Apple's App Store, with a variety of fixes and the addition of "shake to undo," a highly requested feature.

A To-Do List

The main take on the free service is that it turns an e-mail inbox into a to-do list. The app allows users to swipe to archive or delete a message, to read an e-mail thread in a chat-like layout, or to "snooze" an entire inbox and deal with all those messages at some designated later time. Messages can be moved up and down into any desired order. Access to the iPhone's camera is available from within a draft e-mail, to easily take and then add a new photo.

The app's rollout by a company called Orchestra has been phenomenal. Initially by invitation only, within one month of launch the app's volume reached over 60 million e-mails delivered every day. Terms of Dropbox's acquisition of Orchestra were not made public, although there are reports that the price may have been as high as $100 million in cash and stock.

In a posting on Dropbox's blog last week, that company's co-founders, Drew Houston and Arash Ferdowski, wrote that, "like many of you, when we discovered Mailbox we fell in love." They described it as "simple, delightful, and beautifully engineered."

Houston and Ferdowski also mentioned one congruence between the two companies. "Dropbox," they wrote, "doesn't replace your folders or your hard drive: it makes them better." They said the same applies to Mailbox, and that both services wanted "to find ways to simplify your life."

Evolution of Dropbox, E-Mail

Some observers have raised questions about Mailbox's long-term use, such as whether the additional layer on top of Gmail adds more of a target for hackers, and whether Mailbox is vulnerable to changes that Gmail might make.

The acquisition points to two evolutions going on, that of Dropbox and of e-mail. Dropbox, an online storage and file sharing service, has been rapidly adding services to complement what might otherwise become a commodity service -- and it would lose a pricing war for an undifferentiated storage service against such other providers as Google.

In addition, Google has already integrated Gmail into its Google Drive service to facilitate sharing by Gmail users so, in addition to adding value, the Mailbox acquisition is a defensive move by Dropbox.

The Mailbox app also points to the potential and long-delayed evolution of e-mail. As the most popular communications application, e-mail has been viewed as the ancestor of other communication services, but Mailbox is pointing to its use as a task list and possibly other functions.

In that same vein, for instance, IBM announced this week the release of its social e-mail platform, which adds social business functions and essentially makes e-mail the social enterprise hub instead of an add-on.